1. Field of the Invention
The present invention relates to the formation of integrated circuits, and, more particularly, to the formation of contact pads for providing electrical connections to a semiconductor structure.
2. Description of the Related Art
Integrated circuits comprise a large number of individual circuit elements such as, e.g., transistors, capacitors and resistors formed on a substrate. These elements are connected internally by means of electrically conductive lines to form complex circuits, such as memory devices, logic devices and microprocessors. In order to accommodate all the electrically conductive lines required to connect the circuit elements in modern integrated circuits, the electrically conductive lines are arranged in a plurality of levels stacked on top of each other. In order to electrically connect the integrated circuit with other electronic components, input terminals and output terminals are provided.
A method of forming an electrical connection to a semiconductor structure according to the state of the art will now be described with reference to FIGS. 1a–1c. A semiconductor structure 100 comprises a substrate 101 which may be, e.g., a semiconductor wafer on which a plurality of circuit elements and electrically conductive lines connecting the circuit elements have been formed. At a surface of the substrate 102, a contact pad 103 is provided. The substrate 101 comprising the contact pad 103 may be formed by means of advanced techniques of deposition, oxidation, ion implantation and photolithography known to persons skilled in the art.
On the substrate 101, a passivation layer 104 is formed, for example by means of plasma enhanced chemical vapor deposition or chemical vapor deposition. The passivation layer 104 may comprise a dielectric material such as, e.g., silicon nitride, silicon oxynitride or silicon dioxide. The passivation layer 104 prevents moisture from entering the semiconductor structure 100. Thus, electric leakage and corrosion which might lead to a failure of electronic circuits in the semiconductor structure 100 can be significantly reduced.
A mask 105 is formed over the passivation layer 104. The mask 105 does not cover a portion of the passivation layer 104 located over the contact pad 103. The mask 105 may comprise a photoresist. The mask 105, when comprising a photoresist, can be formed by applying the photoresist to the semiconductor structure 100, exposing the photoresist through a reticle and solving either the portions irradiated in the exposure or the non-irradiated portions in a developer.
A schematic sketch of the semiconductor structure in a later stage of the method according to the state of the art is shown in FIG. 1b. After the formation of the mask 105, a first etching process is performed. The first etching process can be a dry etching process. In dry etching, a radio frequency glow discharge produces a chemically reactive species such as atoms, radicals, and ions from a relatively inert molecular gas supplied to a reaction vessel. The reactive species react chemically with the material to be etched, creating a volatile reaction product.
The mask 105 protects portions of the passivation layer 104 covered by the mask 105 from being affected by an etchant used in the first etching process. The portions of the passivation layer 104 over the contact pad 103 which are not covered by the mask 105, however, are etched, and a recess 106 is formed in the passivation layer 104.
The first etching process is stopped prior to a complete removal of the portion of the passivation layer 104 over the contact pad 103. Thus, a residue 107 of the passivation layer 104 remains at the bottom of the recess 106 over the contact pad 103.
Thereafter, the mask 105 is removed by means of a plasma resist stripping process. In the plasma resist stripping process, the semiconductor structure 100 is exposed to a plasma generated by an electric discharge in a gas comprising oxygen. The semiconductor structure is held at a temperature significantly higher than room temperature, for example, 200–300° C. Oxygen radicals in the plasma react chemically with the material of the mask 105, creating a gaseous reaction product which is pumped off.
One purpose of maintaining the residue 107 of the passivation layer 104 over the contact pad 103 is to serve as a protective coating of the contact pad 103 preventing a contact between the oxygen radicals in the plasma and the contact pad 103 in order to avoid an undesirable oxidation of the contact pad 103.
Due to fluctuations in the formation of the passivation layer 104 and/or the etching process performed in the formation of the recess 106, however, a thickness and/or density of parts of the residue 107 of the passivation layer 104 may not be sufficient to prevent a contact between oxygen and the contact pad 103. At the process conditions applied in the conventional resist strip, the contact pad 103, when comprising a metal such as, e.g., copper, oxidizes quickly once it is exposed to oxygen. In particular, the relatively high temperature of 200° C. or more promotes the oxidation of metal. Moreover, since diffusion is a thermally activated process, at high temperature, oxygen diffuses more quickly through permeable portions of the residue 107 of the passivation layer, which further increases the likelihood of an oxidation of the contact pad 103 to occur. Therefore, an oxidized region 108 may form on the surface of the contact pad 103.
A schematic cross-sectional view of the semiconductor structure 100 in a further stage of the manufacturing process according to the state of the art is shown in FIG. 1c. A second etching process is performed in order to remove the residue 107 of the passivation layer 104 from the contact pad 103. The second etching process can be a dry etching process. Conventionally, in the second etching process, an etching gas comprising trifluoromethane (CHF3) and carbon tetrafluoride (CF4) is used. A flow rate of the trifluoromethane is about the same as a flow rate of the carbon tetrafluoride. Thus, an amount of fluorine in the etching gas is greater than an amount of hydrogen in the etching gas. In the second etching process, a polymer layer is formed over a surface of the contact pad 103 which substantially protects the contact pad from being affected by the fluorine in the etching gas.
In the second etching process, or in other steps of the manufacturing process performed thereafter, a flaking of material layers present in the semiconductor structure 100 may occur. This flaking can adversely affect the functionality of the semiconductor structure 100. Moreover, in the first etching process, the removal of the mask and/or the second etching process, contamination layers comprising by-products of chemical reactions, for example polymers formed in the etching processes, may be deposited on the walls of the reaction vessels. Particles may flake off from such contamination layers, in particular in the second etching process, and can be deposited on the semiconductor structure 100, as indicated by reference numeral 109 in FIG. 1c. 
After the second etching process, the contact pad 103 is exposed and may be used to provide electrical contact between the semiconductor structure 100 and other electronic components. For example, this may be done by bonding electrically conductive wires to the contact pad 103 and to other contact pads in the semiconductor structure 100. Alternatively, flip chip methods wherein bumps are formed on the contact pads may be used to bond the semiconductor structure 100 to a circuit board.
A problem of the method of providing electrical contact to a semiconductor structure according to the state of the art is that, in the resist strip process, oxidized regions may form on the contact pad 103. Since the contact pad 103 comprises a metal (copper, for example), and metal oxides, such as copper oxide, are electrical insulators, the presence of the oxidized regions may cause a relatively large contact resistivity between the contact pad 103 and the wire or bump connected thereto. Moreover, oxides on the contact pads 103 may reduce an adhesion between the contact pads and the wires or bumps. This may lead to a reduced stability of the connection.
Another problem of the method of providing electrical contact to a semiconductor structure according to the state of the art is that, in the second etching process, a flaking of layers of the semiconductor structure 100 may occur, which can adversely affect the functionality of the semiconductor structure 100.
Yet another problem of the method of providing electrical contact to a semiconductor structure according to the state of the art is that, in the second etching process, particles comprising by-products of chemical reactions may flake off the walls of the reaction vessels and can be deposited on the semiconductor structure. In order to at least reduce such a deposition of particles, the reaction vessel has to be cleaned frequently in order to remove the by-products from the walls of the reaction vessel. Such cleaning processes, however, increase the downtime of the etching apparatus, which may adversely affect the production costs of semiconductor structures.
In view of the above problems, there is a need for a more reliable method of forming contact pads for providing electrical contact to an integrated circuit.